I'm just offering this out there for anyone that may have the same issue as I did.
The rear indicators/turn signals share the same bulb as the brake lights therefore the turn signals on the rear are red, which is illegal in the UK. I have found the correct modification after much looking through wiring diagrams and using a multimeter.
First off remove the coving from the underside of the steering wheel. This will expose the wiring connections from the turn signal. Where the ribbon of wire becomes single strands is where you will be making some changes. Be careful to cut this wire so you can make connections on both ends, don't cut really close to the connector!
You will find one of the biggest ribbons connects to the multi-function stalk, this is the one you need to work with. It will have many colours. Identify the dark green and yellow wires, also find the white one, but you will use this in a minute.. These need to be cut, remembering to cut them where connections can be made on both ends.
The bare wires that have just come from the turn signal switch are the positive wires for the left and right signals. Yellow is for left and dark green for right. Take these signals and use to connect up new amber rear indicators, and side repeaters which are also required for your MOT test. I mounted the side repeaters on the wing mirror so I didn't have to drill into the bodywork.
You're almost finished. You have sorted the turn signal, but the brake signal still needs to get to the rear lights. Take the white cable and cut it. This supplies the brake signal from the brake switch. Connect the end that is closest to the connector to the yellow and dark green wires that are closest to the connector. This should leave you with only 1 unconnected piece of wire, which is the white one heading to the turn signal switch. Cap this off with some insulation wire.
There you go, that was the tricky part over with.

Finally you need to adjust the corner markers (2 amber at front and 2 red at rear) as you are not allowed to have any red showing to the side of the car. There are 2 ways of doing this. You can either disconnect them by unscrewing them and cutting the wires on the rear (remembering to properly cap them with insulating tape) or the way in which I overcame it was to locate clear replacements for them (I went on eBay). I then put in amber bulbs which are perfectly legal and look nice too.

I don't know much about aligning headlights but the garage can do that in no time.

I hope that may help, I don't want anyone else to have to spend as long as me doing it, or have to spend loads on getting an auto-electrician to do it.

If that has helped you then add a note so I feel appreciated!

 

In case you were wondering I purchased the car from a guy who owned a company supplying cars to the film industry. It is an ex USAF police car we think although graphics tend to change on the cars a load.
I sourced the roof lights from a UK Police car that was about to be crushed and they are fortunately the correct type. It still had the cage and spotlight when I got it and I have reinstated the siren and controller, also sorting out all the knackered wiring.
I hope to get it finished and road legal soon. Here are a couple of pictures /Users/Tony/Pictures/iPhoto Library/Originals/2011/Chevy Fog Light and New Tyres/IMG_2546.JPG
Also the local paper was interested in my work, if you google "cop car Canterbury" you should find it.